Rock bolt with an integrally formed wedge formation and a method of manufacturing same

ABSTRACT

The invention provides an end anchored rock bolt  20  which includes an elongate cylindrical body  12  which longitudinally extends between a proximal end and a distal end and a plug  18  at the distal end, which plug is diametrically larger than the cylindrical body  12,  is at least partially tapered, tapering towards the proximal end, and is integrally formed on the body  12.

FIELD OF THE INVENTION

This invention relates to a rock bolt which is mechanically anchored within a rock hole.

BACKGROUND OF THE INVENTION

Rock bolts generally are used to reinforcement a rock face of an excavation to prevent rock deformation resulting in rock fail from the face. In particular, extensive use is made of rock bolts in underground mining applications to secure the integrity of hanging and side walls and prevent surface sections of these walls from breaking away.

To support a rock face, a rock bolt is inserted into a predrilled rock hole in a wall of a mine excavation. Within the rock hole, however, the rock bolt must be anchored or secured. Typically, this is achieved adhesively, by introducing a resin or adhesive into the rock hole to adhere the rock bolt within the rock hole, or mechanically, using a mechanical anchor engaged with the rock bolt.

Mechanical anchors typically are constructed around an inner tapered plug (wedge element) and an outer expansive shell. The plug is often threadedly engaged to a threaded end section of the rock bolt. Anchoring of the rock bolt within the rock hole occurs when the bolt is rotated, pushed or pulled to cause the plug to be moved into the shell to wedge the shell apart into radially expansive frictional engagement with the rock hole walls.

A problem associated with a mechanical anchor of this type is that the application of a thread to the rock bolt body weakens the structural integrity of the body due to a reduction in the diameter of the body between troughs of the thread.

With the rock bolt body placed in load, the rock bolt body can break along this threaded end section. Also, the plug can be stripped from the rock bolt body. Another possibility is that the blasting vibrations in the mine environment can cause loosening of the threaded engagement of the plug with the rock bolt. In all cases there is a loss of load support of the rock bolt installation.

The invention at least partially addresses the aforementioned problems.

SUMMARY OF INVENTION

The invention provides an end anchored rock bolt which includes an elongate cylindrical body which longitudinally extends between a proximal end and a distal end and a plug at the distal end, which plug is diametrically larger than the cylindrical body, is at least partially tapered, tapering towards the proximal end, and is integrally formed on the body.

The rock bolt may include a radially expansible shell which engages with the body, positioned thereon to radially expand when the plug or the shell is caused to move towards the other.

The radially expansible shell may be biased into abutment with the plug.

The radially expansible shell may comprise a plurality of radially arranged inter-linked leaves.

The plug may have an angle of taper, being a half angle between a longitudinal line and a tapered surface of the plug, in the range 6° to 12°.

Preferably the angle of taper is 9°.

From another perspective, the invention provides an end anchored rock bolt which includes an elongate cylindrical body which longitudinally extends between a proximal end and a distal end and a plug at the distal end and a mechanical anchor, wherein the anchor includes a plug which is integrally formed on the body at the distal end and which is at least partially tapered, tapering towards the proximal end, and a radially expansible shell which engages with the body and is biased into abutment with the plug to radially expand when the plug or the shell is caused to move towards the other.

The invention also provides for a method of manufacturing an end anchored rock bolt, the method including the steps of:

-   -   a) providing an elongate rod of a steel material which         longitudinally extends between a distal end and a proximal end:         and     -   b) forming a plug at the distal end of the rod to be         diametrically larger than the rod and at least partially         tapered, tapering towards the proximal end.

The plug may be formed in a forging process.

The forging process may include at least one upsetting step wherein the distal end of the rod is placed in a first die and a force is directed down the rod so as to cause the distal end of the rod to conform to the shape of the die.

Preferably, the forging process includes a first upsetting step, wherein the distal end of the rod is placed in the first die, and a second upsetting step, wherein the distal end is placed in a second die and wherein, in each step a force is directed down the rod so as to cause the distal end to conform to the shape of the first die and the second die respectively.

The forging process may include a step, preceding the upsetting step, of preheating at least the distal end of the rod before placing it in the at least one die.

Preferably, the distal end of the rod is preheated to a temperature above the recrystallization temperature of the steel material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example with reference to the accompanying drawings in which:

FIG. 1 is an illustration of a leading end of a rock bolt according to the invention and the rock bolt in use;

FIG. 2 is an illustration of a shaft of the rock bolt with an upset end portion;

FIG. 3 illustrates a comparison of the shaft before and after the end portion is upset using two different die; and

FIG. 4 is an illustration of a two-step upsetting method being applied to an end portion of the shaft of the rock bolt.

DESCRIPTION OF PREFERRED EMBODIMENT

Wherever used in this specification, “upsetting” shall refer to any process by which a metal is deformed from its original shape or configuration.

FIG. 1 shows a leading end portion of a rock bolt 10 according to the invention. The rock bolt 10 is to be used to reinforce a mass of rock of a hanging or side wall of, for example, an underground mine. Such reinforcement prevents rock fall from the hanging or side wall.

The rock bolt 10 includes an elongated steel body 12 which extends longitudinally between a distal end 14 and a proximal end (not shown). At the distal end 14, the body has a plug 18 which is diametrically larger than the body and which is wedge or conical-shaped, and which tapers towards the proximal end. The plug is integrally formed with the body as will be more fully described below.

The steel body 12 is a length of high strength hot rolled deformed or smooth steel bar.

The plug forms the core of a mechanical anchor 20, which anchor includes an expansible shell 22, comprising of a plurality of outwardly ridged leaves 24. The leaves are interlinked by a connecting ring 26 to provide a radial array which encircles the body 12 and, at leading ends 28 of the leaves, overlaps the plug.

The rock bolt 10 includes a biasing spring 30 located on the body 12 between the connecting ring 26 and an annular rib 32. The spring provides the biasing force to bias the expansible shell 22 against the plug 18.

A leaf retaining ring 33 holds the expansible shell in an unexpanded or closed configuration as illustrated to the left in FIG. 1.

FIG. 1 also shows, to the right, the rock bolt 10 when put into use. The rock bolt 10 is inserted into a borehole 34, predrilled in a hanging, or side wall of a rock face, the distal end 14 leading. Upon insertion in the hole, a lip o the retaining ring 33 holds the ring back outside of the hole whilst the expansion shell 22 attached to the rock bolt itself continues entry into the hole. Once inserted to optimal depth, the bolt is mechanically anchored therein by pulling longitudinally outwardly on the rock bolt 10 away from the blind end 36 of the hole. Without the retaining ring 33, the shell 22 is free to be driven forwardly relatively to the rock bolt body 12 under action of the biasing spring 30, further over the plug, the taper of which directs the leaves 24 to move radially outwardly into frictional engagement with the borehole walls. Frictional engagement is enhanced by the ridged outer surface of the each of the leaves. In this manner, the rock bolt 10 is mechanically anchored within the borehole.

The plug 18 is an integral part of the body 12 of the rock bolt 10. To make the rock bolt body with the integrally formed plug, a forging process is employed which includes upsetting a portion 38 of the body 12 that terminates at the distal end 14. In this process, the end portion 38 can be shaped into a plug 18 having one of a number of possible configurations, as illustrated in FIGS. 2, 3 and 4.

What each of the differently shaped plugs has in common is that each is diametrically larger than the diameter of the shaft part 40 of the body 12. This is evident in FIG. 2, where d_(o)<d₁ and d_(o)<d₂ However, the volume of the end portion 38 is maintained before and after forming, irrespective of shape.

Although there are a number of alternative forging processes that can be used to achieve the upsetting, it is preferable to use a hot forging process due to the advantages that the process brings. An important advantage of hot forging is that, during deformation of the body 12, the effects of work hardening are negated by the recrystallization process. Cold forging typically results in work hardening. Therefore, due to the amount of material that needs to be moved to integrally form the plug 18 in accordance with the invention, the hot forging process was found, during a process of trial and error, to be the only process that achieved the objective of the invention by industrial production.

Hot forging is defined as a gorging process where the temperature of the work-piece (i.e. the bolt body 12) is above the recrystallization temperature of the composite steel material. If the temperature is below the material's recrystallization temperature but above 30% of the recrystallization temperature (on an absolute scale), it is a warm forging process. If the temperature is below 30% of the recrystallization temperature (usually room temperature) then the process is cold forging.

FIG. 4 illustrates the hot forging process. The preheated end portion 38 is placed in a first die 42, and a longitudinal force, designated F₁, is imposed along the body 12 by ramming or hammering. The end portion 38 will eventually take on the rectangular polyhedral shape of the die 42.

The rectangular end portion 38 is then subject to a further upsetting step using a second die 44 which is conically shaped. The end portion 38 can be heated once again, if necessary, prior to placing the portion in the die and applying a longitudinal force, designated F₂, by hammering or ramming on the bolt body 12. The end portion 38 will thus take on the wedge or truncated conical shape of the die 44.

A further advantage of the hot forging process is that smaller forces (F₁ & F₂) are required to achieve upsetting thus reducing wear on the dies (44, 42).

The following general principles apply when designing upset forged parts:

-   (i) the length of the end portion 38 that can be upset in one blow,     without the formation of integral strength reducing laps, should be     limited to three times the diameter of a shaft part 40, -   (ii) end portion lengths greater than three times the diameter can     be upset successfully, provided that the ultimate diameter of the     end portion (i.e. after upset) is not more than 1.5 times the     diameter of the shaft part 40; and -   (iii) end portion lengths greater than three times the diameter can     be upset successfully in successive upsetting blows, provided the     preceding rules are adhered to per successive blow.

The hot forging process results in improved strength of the end portion 38 as the forces (F₁ & F₂) cause the grain structure of the steel material of the end portion 38 to re-align to conform to the general shape of the die. As a result, the grain structure of the steel material is continuous in curvilinear alignment through the portion, giving rise to improved strength characteristics which is necessary for the plug s 8 to provide the advantages of the invention.

The order of magnitude of material movement necessary to achieve the invention requires the forging process to be hot. This amount of movement, if cold or warm forged, would result in material failure by cracking or even bursting of material.

Use of the rock bolt 10, with the integrally formed plug 18 in accordance with the invention, leads to significant advantages over the rock bolts of the prior art These advantages are: reduced cost of manufacture; increased strength characteristics of the rock bolt; and the capability of the rock bolt to function with a plug which has a diameter which is less than a threadedly engaged tapered nut counterpart found in the prior art.

Threading of the bolt body 12 in order to apply a tapered plug of the prior art reduces the effective cross-sectional area of the body thus reducing its load carrying capacity. Forging the tapered plug 12 onto the body increases the load carrying capacity of the rock bolt 10 about its distal end 14 comparatively with the prior art threaded bolts.

This capability of the rock bolt 10 functioning with an integral tapered plug 18 of lesser diameter, allows use of diametrically smaller boreholes 34. The holes 34 to be used with rock bolt 10 of the invention are smaller in diameter than the holes used with the rock bolts of the prior art, within a 0 mm to 4 mm range of margin on diameter. Drilling smaller holes 34 will result in the following advantages: use of smaller drills. Leading to savings in equipment costs; and less drilling time per hole, leading to an increased rock bolt installation rate and therefore increased mine production.

The geometry necessary for the tapered plug 18 of the invention to achieve its function will inherently result in end portion lengths greater than three times the diameter of the shaft part 40. Therefore, two or three discrete upsetting steps will be required to achieve the end product. A person skilled in the art, knowing that the forging process will require a break from the forging rules mentioned above, would not see forging, and especially hot forging, as an obvious method to form a tapered plug 18 into the bolt body 12. The applicant arrived at the method of the invention to achieve an end anchored rock bolt of the invention, through experimental trial.

The process of hot forging leaves a layer of scale on the surface of the tapered plug 18. This scale is an oxidised iron layer caused by the elevated hot forging temperature. The design of the tapered plug, subsequently, had to be adjusted accordingly by lessening the aggression of the tapered angle which is designated on FIG. 2 (being a half angle between a perpendicular line of the shaft part 40 of the rock bolt body and the taper), which in this example is 9°. This was necessary because an increase in friction was observed between the tapered plug and the leaves 24, as a result of the scale layer. Reducing the taper angle was one way of counteracting the increased frictional interplay to achieve a gripping force onto the walls of the borehole 34 comparable with the prior art type threaded plug.

Subsequent surface treatment of the tapered plug 18, whereby the scale layer is removed either chemically or mechanically, can also be performed to either allow the included taper angle to remain unchanged or compound the effectiveness of the gripping mechanism as a whole. 

1. An end anchored rock bolt which includes an elongate cylindrical body which longitudinally extends between a proximal end and a distal end and a plug at the distal end, which plug is diametrically larger than the cylindrical body, is at least partially tapered, tapering towards the proximal end, and is integrally formed on the body.
 2. An end anchored rock bolt according to claim 1 which includes a radially expansible shell which engages with the body, positioned thereon to radially expand when the plug or the shell is caused to move towards the other.
 3. An end anchored rock bolt according to claim 2 wherein the radially expansible shell is biased into abutment with the plug.
 4. An end anchored rock bolt according to claim 3 wherein the radially expansible shell comprises a plurality of radially arranged interlinked leaves.
 5. An end anchored rock bolt according to claim 1 wherein the plug has an angle of taper in the range 6° to 12°.
 6. An end anchored rock bolt according to claim 5 wherein the angle is 9°.
 7. An end anchored rock bolt which includes an elongate cylindrical body which longitudinally extends between a proximal end and a distal end and a plug at the distal end and a mechanical anchor, wherein the anchor includes a plug which is integrally formed on the body at the distal end and which is at least partially tapered, tapering towards the proximal end, and a radially expansible shell which engages with the body and is biased into abutment with the plug to radially expand when the plug or the shell is caused to move towards the other.
 8. A method of manufacturing an end anchored rock bolt, the method including the steps of: a) providing an elongate rod of a steel material which longitudinally extends between a distal end and a proximal end; and b) forming a plug at the distal end of the rod to be diametrically larger than the rod and at least partially tapered, tapering towards the proximal end.
 9. A method according to claim 8 wherein the plug is formed in a forging process.
 10. A method according to claim 9 wherein the forging process includes at least one upsetting step, wherein the distal end of the rod is placed in a first one die and a force is directed down the rod so as to cause the distal end of the rod to conform to the shape of the die.
 11. A method according to claim 10 wherein the forging process includes a first upsetting step, wherein the distal end of the rod is placed in the first die, and a second upsetting step wherein the distal end is placed in a second die, and wherein, in each step a force is directed down the rod so as to cause the distal end to conform to the shape of the first die and the second die respectively.
 12. A method according to claim 10 wherein the forging process includes a step, preceding the upsetting step or steps of preheating the distal end of the rod before placing it in the respective die.
 13. A method according to claim 12 wherein the distal end of the rod is preheated to a temperature above the recrystallization temperature of the steel material. 